I think that if the kindle settings are left on the "set automatically" default selection, then they will change when you connect via 3G or Wi-Fi.
On a 3G connection, "set automatically" may adjust timezone and time from 3G packet data (visible in settings 611 page 2).
On a Wi-Fi connection, "set automatically" may adjust timezone from your online Amazon profile and time from an NTP server.
Manual time settings may only stick if you select the "edit / set manually" setting, after which you may adjust the time however you wish.
NiLuje concurs:
https://www.mobileread.com/forums/sho...76&postcount=4